Add support for persistent sparse histograms.

base/metrics/sparse_histogram.cc

Index: base/metrics/sparse_histogram.cc
diff --git a/base/metrics/sparse_histogram.cc b/base/metrics/sparse_histogram.cc
index 5653456a29fb552470871f8611cb6337ec65bc50..978514bbce03c46b74606faea4e52b823f082ecd 100644
--- a/base/metrics/sparse_histogram.cc
+++ b/base/metrics/sparse_histogram.cc
@@ -6,6 +6,7 @@
 
 #include <utility>
 
+#include "base/metrics/histogram_persistence.h"
 #include "base/metrics/metrics_hashes.h"
 #include "base/metrics/sample_map.h"
 #include "base/metrics/statistics_recorder.h"
@@ -21,23 +22,73 @@ typedef HistogramBase::Sample Sample;
 // static
 HistogramBase* SparseHistogram::FactoryGet(const std::string& name,
                                            int32_t flags) {
-  HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
+  // Import histograms from known persistent storage. Histograms could have
+  // been added by other processes and they must be fetched and recognized
+  // locally in order to be found by FindHistograms() below. If the persistent
+  // memory segment is not shared between processes, this call does nothing.
+  ImportPersistentHistograms();
 
+  HistogramBase* histogram = StatisticsRecorder::FindHistogram(name);
   if (!histogram) {
-    // To avoid racy destruction at shutdown, the following will be leaked.
-    HistogramBase* tentative_histogram = new SparseHistogram(name);
+    // Try to create the histogram using a "persistent" allocator. As of
+    // 2016-02-25, the availability of such is controlled by a base::Feature
+    // that is off by default. If the allocator doesn't exist or if
+    // allocating from it fails, code below will allocate the histogram from
+    // the process heap.
+    PersistentMemoryAllocator::Reference histogram_ref = 0;
+    HistogramBase* tentative_histogram = nullptr;
+    PersistentMemoryAllocator* allocator =
+        GetPersistentHistogramMemoryAllocator();
+    if (allocator) {
+      flags |= HistogramBase::kIsPersistent;
+      tentative_histogram = AllocatePersistentHistogram(
+          allocator,
+          SPARSE_HISTOGRAM,
+          name,
+          0,
+          0,
+          nullptr,
+          flags,
+          &histogram_ref);
+    }
+
+    // Handle the case where no persistent allocator is present or the
+    // persistent allocation fails (perhaps because it is full).
+    if (!tentative_histogram) {
+      DCHECK(!histogram_ref);  // Should never have been set.
+      DCHECK(!allocator);      // Shouldn't have failed.
+      flags &= ~HistogramBase::kIsPersistent;
+      tentative_histogram = new SparseHistogram(name);
+    }
+
     tentative_histogram->SetFlags(flags);
     histogram =
         StatisticsRecorder::RegisterOrDeleteDuplicate(tentative_histogram);
+
+    // Persistent histograms need some follow-up processing.
+    if (histogram_ref) {
+      FinalizePersistentHistogram(histogram_ref,
+                                  histogram == tentative_histogram);
+    }
   }
+
   DCHECK_EQ(SPARSE_HISTOGRAM, histogram->GetHistogramType());
   return histogram;
 }
 
+// static
+HistogramBase* SparseHistogram::PersistentGet(
+    PersistentMemoryAllocator* allocator,
+    const std::string& name,
+    HistogramSamples::Metadata* meta,
+    HistogramSamples::Metadata* logged_meta) {
+  return new SparseHistogram(allocator, name, meta, logged_meta);
+}
+
 SparseHistogram::~SparseHistogram() {}
 
 uint64_t SparseHistogram::name_hash() const {
-  return samples_.id();
+  return samples_->id();
 }
 
 HistogramType SparseHistogram::GetHistogramType() const {
@@ -63,7 +114,7 @@ void SparseHistogram::AddCount(Sample value, int count) {
   }
   {
     base::AutoLock auto_lock(lock_);
-    samples_.Accumulate(value, count);
+    samples_->Accumulate(value, count);
   }
 
   FindAndRunCallback(value);
@@ -73,29 +124,29 @@ scoped_ptr<HistogramSamples> SparseHistogram::SnapshotSamples() const {
   scoped_ptr<SampleMap> snapshot(new SampleMap(name_hash()));
 
   base::AutoLock auto_lock(lock_);
-  snapshot->Add(samples_);
+  snapshot->Add(*samples_);
   return std::move(snapshot);
 }
 
 scoped_ptr<HistogramSamples> SparseHistogram::SnapshotDelta() {
   scoped_ptr<SampleMap> snapshot(new SampleMap(name_hash()));
   base::AutoLock auto_lock(lock_);
-  snapshot->Add(samples_);
+  snapshot->Add(*samples_);
 
   // Subtract what was previously logged and update that information.
-  snapshot->Subtract(logged_samples_);
-  logged_samples_.Add(*snapshot);
+  snapshot->Subtract(*logged_samples_);
+  logged_samples_->Add(*snapshot);
   return std::move(snapshot);
 }
 
 void SparseHistogram::AddSamples(const HistogramSamples& samples) {
   base::AutoLock auto_lock(lock_);
-  samples_.Add(samples);
+  samples_->Add(samples);
 }
 
 bool SparseHistogram::AddSamplesFromPickle(PickleIterator* iter) {
   base::AutoLock auto_lock(lock_);
-  return samples_.AddFromPickle(iter);
+  return samples_->AddFromPickle(iter);
 }
 
 void SparseHistogram::WriteHTMLGraph(std::string* output) const {
@@ -114,7 +165,19 @@ bool SparseHistogram::SerializeInfoImpl(Pickle* pickle) const {
 
 SparseHistogram::SparseHistogram(const std::string& name)
     : HistogramBase(name),
-      samples_(HashMetricName(name)) {}
+      samples_(new SampleMap(HashMetricName(name))),
+      logged_samples_(new SampleMap(samples_->id())) {}
+
+SparseHistogram::SparseHistogram(PersistentMemoryAllocator* allocator,
+                                 const std::string& name,
+                                 HistogramSamples::Metadata* meta,
+                                 HistogramSamples::Metadata* logged_meta)
+    : HistogramBase(name),
+      samples_(new PersistentSampleMap(HashMetricName(name), allocator, meta)),
+      // The ID must be different so as to not confuse sample records.
+      logged_samples_(
+          new PersistentSampleMap(samples_->id() + 1, allocator, logged_meta)) {
+}
 
 HistogramBase* SparseHistogram::DeserializeInfoImpl(PickleIterator* iter) {
   std::string histogram_name;